1. Field of the Invention
The present invention relates to an apparatus for converting an image frame rate, and more particularly, to a method and apparatus for interpolating an image that improve frequency characteristics of a motion compensated interpolation by combining motion compensation and a de-blurring filter.
2. Description of the Related Art
Generally, personal computers (PCs) or high definition televisions (HDTVs) perform frame rate conversion in order to interchange programs according to various broadcasting signal specifications such as Phase Alternating Line (PAL) or National Television System Committee (NTSC). The frame rate conversion is for converting the number of frames that are output per second. In particular, a process for interpolating a new frame between frames is needed when a frame rate increases.
Most film image signals include 24 fps to show movies, whereas image signals include 30 fps according to an NTSC specification. Thus, film image signals must include 30 frames (or 60 field/sec) so as to apply them to an NTSC specification format.
FIG. 1 is a diagram for explaining a conventional frame rate converting method from a 24 fps digital image signal to a 30 fps digital image signal. Referring to FIG. 1, new frames F1′ through F4′ are generated by coping given frames F1 through F4. An interpolation frame Fi is generated by applying the given frames F2 and F3 to motion compensation having a certain rate. In more detail, a motion compensated frame by half a motion vector in a direction from the second frame F2 to the third frame F3 and a motion compensated frame by half a motion vector in a direction from the third frame F3 to the second frame F2. The combined frame is inserted into the interpolation frame F1 between the second frame F2 and the third frame F3.
In this regard, when sub-pixel motion estimation is used to interpolate an image or a frame, an interpolation area obtained by the motion compensation from a previous or next frame is slightly different from a substantial interpolation area.
In more detail, a pixel that is to be interpolated is obtained from a pixel of previous and next frames by dividing a motion vector value obtained by the motion estimation at a certain rate. The divided motion vector value is an integer value or a decimal value. If the motion vector value is not the integer value as shown in FIG. 2 that is a diagram for explaining a conventional image interpolation method, the pixel that is to be interpolated obtains, from the previous and next frames, a value containing a decimal point.
However, since a corresponding pixel is not substantially included in the previous and next frames at a location x of the motion vector that contains the decimal point, the interpolated pixel is obtained from four pixel values P1 through P4 around the location x of the motion vector by performing a well-known linear or non-linear operation.
However, the four pixels P1 through P4 correspond to values located in an integer part, and a sub-pixel value of the location x of the motion vector corresponds to a value located in a decimal part. Thus, an image detail difference occurs when the values located in the integer and decimal parts are compensated.
The conventional pixel interpolation method low-passes a frequency component of a pixel by performing the linear or non-linear operation, causing blurring. That is, an interpolation pixel according to the size of a motion vector and a motion compensation rate of previous and next frames may lose a high frequency component.